Not entirely sure if it goes to account creation (which I hope it does/can), but this might work.

Edit: It does!There are five "triggers" on which CAPTCHAs can be displayed:'edit' - triggered on every attempted page save'create' - triggered on page creation'addurl' - triggered on a page save that would add one or more URLs to the page'createaccount' - triggered on creation of a new account'badlogin' - triggered on the next login attempt after a failed one. Requires $wgMainCacheType to be set to something other than CACHE_NONE.